The present invention relates to transducers, and particularly to electrostatic loudspeakers for the reproduction of music.
A conventional electrostatic loudspeaker is normally composed of a thin membrane, or diaphragm, made of Mylar, or the like that is stretched between two acoustically open wire grids, or plates. The latter plates are connected to a coupling transformer which provides a high voltage audio signal, and the diaphragm is connected to a high voltage, low current bias supply which provides an electrostatic charge that becomes trapped in the diaphragm.
The two acoustically open plates provide an electric field that is the voltage equivalent of the audio signal and which exerts forces on the electrostatic charge that is trapped in the diaphragm. These forces are transferred to the diaphragm causing the diaphragm to move in synchronization with the forces and reproduce the input signal.
The electrostatic loudspeaker enjoys several advantages when compared to normal dynamic speakers consisting of a frame housing a magnet and a voice coil attached to the apex of a cone which is suspended at the edge by a flexible cloth or the like. For example, the moving member, i.e., the moving diaphragm of the electrostatic loudspeaker is very thin and light (i.e. its thickness is usually only 0.0002.+-.0.000025 inches and it weighs only as much as a body of air 7 millimeters thick whose boundaries are equal to those of the moving diaphragm). Also, the electric field which acts to make the diaphragm move exerts its actuating force uniformly over essentially the entire area of the diaphragm. A diaphragm of such extreme lightness in combination with the uniformly distributed actuating force results in a diaphragm motion that is a very good replica of the electrical forces acting upon it. In addition, all sections of the diaphragm surface move with highly accurate phase and amplitude linearity throughout its entire range of travel at all frequencies within its area of operation.
Another advantage of the electrostatic transducer is that it is inherently a unit with low mechanical impedance at all frequencies. Thus, it couples to the air with reasonable efficiency at all frequencies which is not necessarily the case for dynamic units which are encumbered by a relatively high mechanical impedance. As a result, the electrostatic transducer performs well down to its frequency limits and within its maximum excursibility with virtually equal fidelity at all drive levels.
However, two potential problems do result in the design and use of an electrostatic transducer. First of all, at the resonant frequency a response peak is exhibited by the stretched, under-damped diaphragm which is very responsive and dynamic. Therefore, the diaphragm tends to "slap" the plates with very little provocation, since every unit of diaphragm area contributes its parcel of energy to the energy peak.
Attempts have been made to eliminate the resonant peak mentioned above. However, these attempts usually dissipate the energy of the peak without putting it to work and, in addition, restricts the dynamic range not at just the peak frequency but over an adjacent range of frequencies.
Also, the vibrating diaphragm of an electrostatic transducer is normally permitted to propagate freely in its environment and, as such, has acoustic energy emanating from both of its sides to minimize colorations in the reproduced signal. However, since the two waves radiating from the two sides of the diaphragm are mutually out of phase they begin to cancel one another at lower frequencies where acoustic wavelengths are longer than the physical dimensions of the speaker. This results in poor bass response at the lower frequencies.